The invention described herein relates generally to methods and apparatus for producing magnified X-ray images of X-ray emitting objects.
J. W. M. DuMond and Harry A. Kirkpatrick, Rev. Sci. Instr. 1, 88 (1930), discuss the problem of finding the contour to which a flexible crystal surface must conform so that monochromatic X-ray radiation from a point A would be selectively focused by Bragg reflection at a point B. Bragg reflection imposes two conditions at every point on the curved crystal surface:
(1) At all points on the surface the angles of incidence and reflection, referred to the reflecting atomic planes, must be equal. PA1 (2) At all points on the surface the angle of deviation of the reflected beam must be constant.
These two conditions dictate the position and the slope of every point on the curved surface of the flexible crystal. In the usual case, where the atomic crystal planes are locally parallel to the reflecting boundary of the crystal surface, no continuous smooth surface contour can simultaneously satisfy these two conditional parameters. However, noticing that condition (1) dictates the direction of the atomic reflecting planes but imposes no condition on the reflecting boundary of the crystal, and that condition (2) dictates the position of every point on the reflecting boundary but demands nothing of the atomic reflecting planes, the two conditional parameters can, in fact, be simultaneously satisfied by crystal configurations wherein the atomic reflecting planes are not required to be parallel to the reflecting boundary of the crystal. Dumond and Kirkpatrick then proceed to disclose that, in a cylindrical situation, by employing a crystal whose reflecting boundary coincides with part of the outer surface of a circle, and whose atomic reflecting planes are bent to coincide with concentric circles centered on a point on the circumference of the circle that is diametrically across the circle from the crystal, the two Bragg reflection focusing conditions can be simultaneously met.
Johansson, Zeitschrift fur Physik 82, 507 (1933), develops the reflecting geometry of Dumond and Kirkpartrick, which has come to be known as the Johansson curved-crystal dispersion arrangement. As a practical matter, Johansson spectrometers employ circularly cylindrical crystal surfaces and atomic reflecting bent planes, so that points are focused approximately to lines, which is ideal for X-ray line spectroscopy.
Spherically curved point-focusing Bragg monochrometers, wherein a crystal is spherically bent to a radius twice that of the focal circle and then ground so that the front surface is spherical and of the same radius as the focal circle, that are extensions of the Johansson geometry, have been discussed by Ehrhardt et al, Applied Spectroscopy 22, 730 (1968).
The crystals used by Ehrhardt et al, supra, were bent at elevated temperatures by extensions of a technique suggested by Birks et al, Rev. Sci. Instr. 24, 992 (1953), wherein an ordinary tennis ball may be used to form a flexible concave die in the bending process.
It should be noticed that the discussion has thusfar been limited to the point- or line-focusing of monochromatic, single wavelength, X-rays.
The formation of chromatic optical images by X-rays and the possibility of constructing an X-ray microscope were discussed by Paul Kirkpatrick and A. V. Baez, J. Opt. Soc. Amer. 38, 766 (1948). They point out that two internal total reflection at small grazing angles X-ray mirrors may be positioned to produce point images of point objects, and therefore real, extended images of extended objects. They suggest, without elaboration, that elliptical and parabolic surfaces will almost certainly be superior to spherical surfaces for this purpose.
Wolter, in U.S. Pat. No. 2,759,106 issued Aug. 14, 1956 and claiming priority from a German application filed May 25, 1951, discloses an X-ray optical image-forming mirror system that comprises hyperboloid and ellipsoid small grazing angle reflecting surfaces having a common axis. Wolter also, in a beautiful paper, Annalender Physik 10, 94 (1952), discusses X-ray optics closely related to his patented hyperboloid-ellipsoid system. An embodiment of this Wolter small grazing angle mirror system has been built and operated at the Lawrence Livermore National Laboratory; it is described by Boyle et al in Rev. Sci. Instr. 49, 746 (1978).
Keem et al, U.S. Pat. No. 4,525,853 issued June 25, 1985 teaches a point source non-imaging X-ray focusing device wherein the focusing element comprises the inner surface of an ellipsoid with a synthetic multilayer formed thereupon. The layer pairs of the multilayer are locally parallel to the surface boundary of the ellipsoid. The source and focus are at the foci of the ellipsoid. The synthetic multilayer coating can be thickness graded to retain reflectance over increased portions of the surface of the ellipsoid by compensating for the change in incident angle at different locations on the reflecting surface.
It is thus observed that all prior art methods and apparatus for producing magnified chromatic X-ray images of extended X-ray emitting objects, rely on and are limited to techniques that utilize small grazing angle total internal X-ray reflection.